Method for making multimedia storage system with highly compact memory cells

ABSTRACT

A highly compact nonvolatile solid state memory core is provided that stores and reproduces both digital and analog signals for multimedia applications. The memory core includes vertical electrically erasable and programmable read only memories (EEPROM) cells having, for example, a stacked gate or a split channel configuration. An array of EEPROM cells on the same chip is prewritten and is used as a reference for digital-analog conversions and for memory cell programming. An intelligent write method allows each memory cell to either store an analog signal or multiple digital signals. Based on the previously stored signal, the intelligent write method determines whether to charge or to discharge the floating gate associated with the selected memory cell. Thus, full erasure is not required prior to programming each memory cell. The present invention significantly increases the density of memory cell arrays while prolonging the useful life of the array.

This application is a division of application Ser. No. 08/336,361, filedNov. 8, 1994 which was a continuation of application Ser. No.07/970,728, now U.S. Pat. No. 5,386,132, filed Nov. 2, 1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to memory cells, and particularly to electricallyerasable and programmable read only memories (EEPROMs) used asnonvolatile semiconductor storage systems to store both digital andanalog signals.

2. Description of Related Art

As is well known in the art, an EEPROM cell utilizes a floatingpolysilicon layer, conventionally called a floating gate, positionedbetween but insulated from a channel region and a control gate. Thechannel region is formed between a source and a drain region. Thefloating gate retains charges, thereby varying the threshold voltage ofthe transistor. The threshold voltage is the minimum voltage applied tothe control gate to turn on the transistor, i.e. to allow current toflow between source and drain regions. Each EEPROM cell conventionallystores only one bit of data, i.e. either a "0" or a "1". For example, astored "0" in one convention is associated with an "on" state while astored "1" is associated with an "off" state.

To read the data stored in a memory cell, a voltage is applied acrossthe source and drain regions while another voltage is placed on thecontrol gate. The amount of current flowing between source and drainregions as a result of these applied voltages determines whether thetransistor is "on" or "off". In the "on" state, the channel isconducting and current flows between the source and drain regions,whereas in the "off" state, the channel is not conducting and no currentflows between the source and drain regions. A memory cell can beprogrammed to the "on" or "off" state by changing the amount of chargeretained on the floating gate so that the threshold voltage is eitherhigher than the applied control gate voltage (resulting in an "off"state) or lower than the applied control gate voltage (resulting in an"on" state).

Writing data into a memory cell typically includes two steps. First, thecell is erased by removing charges from the floating gate, and then thecell is programmed by charging the floating gate according to the datato be stored. Transferring charges to and from the floating gate isachieved either by hot electron injection or tunnelling. During hotelectron injection, electrons, accelerated in a high electrical fieldacross the channel region, acquire the energy necessary to overcome theenergy barrier imposed by the insulation formed between the channelregion and the floating gate. During tunneling, electrons drift acrossthe energy barrier of the insulation formed between the floating gateand other nearby electrodes, e.g. the source region, the drain region,or the channel region, by applying a high electrical field across theinsulation. A conventional EEPROM cell usually relies on tunneling forboth programming and erasing operations. To ensure cell-by-cellerasability and to avoid read errors caused by over-erase of some of theunselected cells, a separate select transistor is typically used todisconnect the drain regions of the unselected cells from the selectedcell during erase and read operations. However, this additionaltransistor increases the size of the memory cell array.

A common trend in the semiconductor memory industry is to reduce thesize of the memory array to achieve higher cell density on a singlechip, thereby lowering the cost per bit. One method of reducing thearray size is to eliminate the select transistors, as disclosed byMasuoka, et al. in an article entitled, "A new flash EPROM cell usingtriple polysilicon technology", IEDM Proceedings, page 464, 1984.Masuoka et al. reduced array size by using a split channel structureshown in FIGS. 1A and 1B and a flash erase scheme. A split-channelstructure is conventionally defined as two transistors, a floating gatestorage transistor and a select transistor, sharing a channel region.The term "flash" refers to the fact that an entire array or a relativelylarge block of memory cells is erased simultaneously.

FIGS. 1A and 1B show cross-sectional views of the memory cell 100 ofMasuoka et al. along a bit line and a word line, respectively. Referringto these figures, the memory cell 100 erases using electron tunnelingfrom floating gate 101 to a separate erase gate 102. Erase gate 102eliminates the problem of having a very high voltage on source region105 or drain region 106. However, the amount of overlap between erasegate 102 and floating gate 101 is critical, thereby increasingmanufacturing complexity and lowering yield.

Advances in methods of growing oxide have allowed thinner layers ofoxide to be used as insulation, thereby reducing the required thresholdvoltage for tunneling. FIG. 2 illustrates a stacked gate EEPROM cell 200disclosed by Mukherjee et al. in an article entitled "A SingleTransistor EEPROM Cell And Its Implementation In A 512K CMOS EEPROM"IEDM Proceedings, page 616, 1985. Mukherjee et al. eliminated the needfor erase gate 102 (FIG. 1B) by allowing electrons to tunnel fromfloating gate 201 to source region 205. The thin insulation layer 204facilitates this tunnelling. However, because electrical erasing is notself-limiting, floating gate 201 is likely to be over-erased and becomepositively charged after erasing. This positive charge on floating gate201 turns memory cell 200 into a depletion mode transistor. If thefloating gate transistor becomes a depletion-mode transistor after beingover-erased, the transistor turns on even if it is unselected, therebycausing read errors of the selected cell.

To overcome this over-erase problem, G. Samachisa, et al. disclosed asplit channel structure in an article entitled, "A 128K Flash EEPROMusing Double-Polysilicon Technology", IEEE Journal of Solid StateCircuits, Vol. SC-22, No. 5, page 676, October 1987. Referring to FIG.3, Samachisa et al. added a series select transistor 307 to stacked gateEEPROM cell 200 to control the channel conductance. In this manner, ifseries select transistor 307 is off, no current flows between the sourceregion 305 and the drain region 306, even if the floating gatetransistor 300 is in a depletion mode. Like cell 200, cell 300 ofSamachisa has a thin gate insulation 304 to allow electron tunnelingfrom floating gate 301 to drain region 306.

Another method to overcome the over-erase problem was disclosed by V. N.Kynett, et al. in an article entitled, "An In-System Reprogrammable32K×8 CMOS Flash Memory", IEEE Journal of Solid State Circuits, Vol. 23,No. 5, page 1157, October 1988. Kynett added program and erase verifycircuitry (not shown) to the stacked gate flash-EEPROM cell 200.However, Kynett, like Mukherjee et al. and Samachisa et al. relies onthe thinness of the gate insulation for electron tunneling, therebycreating problems in reliability and manufacturing yield.

Therefore, other methods to reduce array size, such as storing more thanone bit of data per memory cell, have been devised. The concept ofstoring multiple bits of data per memory cell is essentially the same asstoring an analog signal in a memory cell. Due to the advancement indigital signal processing, an analog signal is often converted into andrepresented by a string of binary code or a word of multiple bits. Thehigher the number of bits representing a sampled and held analog signal,the higher the accuracy of the signal represented. Thus, an EEPROM cellthat stores multiple bits of data can effectively store a completesampled and held analog signal. The accuracy of reproducing a storedanalog signal typically depends on the full range of cell current andthe signal to noise ratio.

An EEPROM cell, disclosed by Bleiker et al. in an article entitled "AFour-State EEPROM Using Floating-Gate Memory Cells", IEEE Journal OfSolid State Circuits, Vol. SC-22, No. 3, pages 460-463, June 1987,stored two bits of data. The four distinguishable states of this cellwere defined by the levels of cell current detected. Each state wascharacterized by an upper and lower current limit. However, as is wellknown to those skilled in the art, to accurately detect the data storedin the memory cell within a reasonable read time, the current level usedto characterize the least significant bit (LSB) must be large enough toyield an acceptable signal to noise ratio. Moreover, for a given LSB,the range of cell current must be increased to store additional bits ofdata per memory cell.

To increase cell current without changing the channel length, prior artmethods typically increased the transistor width. However, an increasein transistor width increases memory cell size for conventional planarcells (such as cells 100, 200, and 300), i.e. cells which have source,drain and gate regions above and/or near the surface of the substrate.

To decrease memory cell size while providing increased cell current,Yoshida, et al. disclosed a vertical EPROM/EEPROM cell in U.S. Pat. No.5,049,956, issued Sep. 17, 1991. Referring to FIG. 4 of thisspecification, a vertical stacked gate EEPROM cell 400 is formed on theside wall 406 of a trench 407 etched to source region 402 which isburied in substrate 401. The width of cell 400, which equals theperimeter of trench 407, is roughly four times that of planar cell 200(FIG. 2) but occupies the same silicon area. However, this structure,like cell 200, lacks a select transistor and thus has no cell-by-cellerasability. Moreover, the thin gate insulation 408 which overlies theentire channel region 403 induces reliability and yield problems.Further, the uneven topography of cell 400, created by the projection offloating gate 405 above the surface of the array, further worsensreliability and yield.

Therefore, a need arises for a reliable memory cell capable of storingmultiple data bits while occupying a small surface area of the chip.

SUMMARY OF THE INVENTION

In accordance with the present invention, a vertical memory cell uses apredetermined surface area of a chip regardless of the featuresintegrated in the cell. The vertical memory cell comprises a buriedsource region, a drain region, a channel region formed between thesource and drain regions, a floating gate insulated from the drain,channel, and source regions, and a control gate insulated from thefloating gate.

In one embodiment of the present invention, the floating gate includes afirst and a second portion, wherein each portion is spaced independentlyfrom the drain region, i.e. the insulation between the first portion andthe drain region is formed thicker than the insulation between thesecond portion and the drain region. During erase operations, electronstunnel between the second portion of the floating gate, which overliesthe drain region, and the drain region. The reduced tunnel areaassociated with the second portion provides significant improvements inreliability and yield over prior art cells which provide tunnel oxideadjacent the whole channel region.

In another embodiment of the present invention, the drain regionincludes an extended drain region to provide a tunneling electricalfield enhancement effect. This extended drain region is made frompolysilicon, for example, which lowers the voltage required to achievetunneling from the floating gate to the drain region. Thus, thisembodiment also improves manufacturing yield by allowing for thickerinsulation to be used between the floating gate and the channel region.

In yet another embodiment, the vertical memory cell further includes anerase/program gate positioned between the drain region and the controlgate, and in operative relation to the floating gate and the source anddrain regions. A predetermined voltage bias on the control gate and theerase/program gate allows electrons to tunnel between the erase/programgate and the floating gate during program and erase operations. In thismanner, the present invention provides cell-by-cell erasing andprogramming.

In another embodiment of the present invention, the vertical memory cellcomprises a first transistor formed in a first trench in a substrate anda second transistor formed in a second trench in the substrate. Thefirst and second trenches are aligned, thereby forming a split channelstructure. The first transistor is typically a floating gate storagetransistor, while the second transistor is typically a series selecttransistor. The series select transistor eliminates any potential readerrors caused by an over-erased floating gate. Specifically, if theseries select transistor is off, no current flows between the sourceregion and the drain region, even if the floating gate transistor is ina depletion mode.

Additional features, such as the segmented floating gate, the extendeddrain region, and the erase/program gate, are incorporated into thissplit channel structure in other embodiments without increasing therequired surface area of the chip.

In further accordance with the present invention, the above-describedvertical memory cells form a mass storage system that accepts eitherdigital or analog input signals and reproduces the stored information ineither form. This mass storage system includes a plurality of verticalmemory cells which are prewritten to store a set of references forwriting, reading, and digital-analog conversions. The number ofreferences required for storing n bits of data per memory cell is 2^(n)+1. The state of each cell is characterized by two references: an upperlimit and a lower limit.

For each digital input signal, a pair of references is selectedaccording to the state that represents the input signal. Based on thisset of references, the voltage of the erase/program gate and the controlgate are varied to perform the appropriate erase and program functionssuch that the input signal is written into a memory cell. For eachsampled and held analog input signal S_(a), the set of references usedto vary voltages for writing is S_(a) ± 1/2 LSB, where LSB (the leastsignificant bit) is the difference between two adjacent states.

To output stored information as an analog signal, the memory cellcurrent is provided to one input of a differential amplifier. Areference level of maximum channel conducting is provided to the otherinput to the differential amplifier to compensate for variations in theperformance of the storage cell due to temperature and supply voltagefluctuations. The output signal of the differential amplifier is theanalog output of the memory cell.

To output stored information as a digital signal, the cell current orbit line voltage is fed to a set of comparators to compare with eachreference in parallel. Then, the output signals from the comparators areencoded to provide the output digital data.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show cross-sectional views of the split channelflash-EEPROM cell of Masuoka with separate erase gate along a bit lineand a word line, respectively.

FIG. 2 illustrates a cross-sectional view of the stacked gateflash-EEPROM cell of Mukherjee et al. along a bit line.

FIG. 3 shows a cross-sectional view of the split channel Flash-EEPROMcell of Samachisa et al. along a bit line.

FIG. 4 illustrates a cross-sectional view along a bit line of theEPROM/EEPROM cell of Yoshida et al. formed in a trench.

FIGS. 5A and 5B show cross-sectional views of a vertical stacked gateEEPROM cell along a word line and a bit line, respectively.

FIG. 6 illustrates a cross-sectional view along a word line of avertical stacked gate EEPROM cell with tunnel oxide formed on the topsurface of the drain region.

FIGS. 7A and 7B show cross-sectional views of a vertical stacked gateEEPROM cell with an extended drain region along a word line and a bitline, respectively.

FIGS. 8A and 8B illustrate cross-sectional views of a vertical stackedgate EEPROM cell with a separate erase/program gate along a word lineand a bit line, respectively.

FIGS. 9A and 9B show cross-sectional views of a vertical split channelEEPROM cell with a separate erase/program gate along a word line and abit line, respectively.

FIGS. 10A through 10E illustrate cross-sectional views along a word lineof two vertical split channel EEPROM cells with separate erase/programgates at various stages in fabrication in a non-self-aligned process.

FIG. 11 shows a top view of a 2×2 array of vertical EEPROM cell formednon-self-aligned to the edges of bit line isolation.

FIGS. 12A through 12D illustrate cross-sectional views along a word lineof two vertical split channel EEPROM cells with separate erase/programgates at various stages in fabrication in a self-aligned process.

FIG. 13 shows a top view of a 2×2 array of vertical EEPROM cell formedself-aligned to the edges of bit line isolation.

FIGS. 14A through 14E illustrate cross-sectional views along a word lineof two vertical split channel EEPROM cells with tunnel oxide formed onthe top surface of the separate erase/program gate at various stages ina non-self-aligned process fabrication.

FIG. 15 shows a cross-sectional view along a word line of a verticalsplit channel EEPROM cell with a separate erase/program gate formed on astandard CMOS N-type epitaxial layer on an antimony doped N⁺ siliconsubstrate.

FIG. 16 shows a schematic diagram of a 2×2 array of vertical splitchannel EEPROM cell with separate erase/program gates.

FIG. 17 illustrates a timing diagram of a selected memory cell.

FIG. 18 shows a schematic diagram showing how each digital data isrepresented by a pair of references.

FIG. 19 illustrates a schematic diagram of a write circuit using upperand lower limits to steer the programming and erasure of a selectedstorage cell.

FIG. 20 shows a schematic diagram of a read circuit using comparators inparallel to encode the comparison results to digital output.

FIG. 21 illustrates a schematic block diagram of a system for storingmultiple bits per cell.

FIG. 22 shows a perspective cross-sectional drawing of an array ofvertical stacked gate EEPROM structures formed on the side walls of longopen trenches.

FIGS. 23A through 23C illustrate cross-sectional views of verticalstacked gate EEPROM structures along a word line formed on sidewalls oflong open trenches at various processing stages in fabrication.

FIG. 23D shows a cross-sectional view of vertical split channel EEPROMstructures along a word line formed on sidewalls of long open trenches.

FIG. 24 illustrates a top view of an array of vertical EEPROM cellsformed on the side walls of long open trenches.

FIG. 25A illustrates a schematic diagram of an array of vertical EEPROMcells formed on side walls of long open trenches having a common source.

FIG. 25B is a schematic diagram of an array of vertical EEPROM cellsformed on the side walls of long open trenches having an alternatesource.

DETAILED DESCRIPTION OF THE DRAWINGS

I. Vertical EEPROM Cells

In accordance with the present invention, all vertical EEPROM cells takeup identical surface area on the chip regardless of the differentfeatures integrated into each cell. These features include, for example,a floating gate which partially overlies the drain region, an extendeddrain region, a separate erase/program gate, and a series selecttransistor.

1. Vertical Stacked Gate EEPROM

FIGS. 5A and 5B show cross-sectional views of a vertical stacked gateEEPROM transistor 500 in accordance with the present invention along aword line and a bit line, respectively. As shown in FIGS. 5A and 5B, achannel region 503 is formed on top of a source region 502, and a drainregion 504 is formed on top of the channel region 503. Drain regions 504of memory cells sharing the same bit line are connected by N+ diffusion,thereby eliminating the need for metal to diffusion contacts along thebit line.

Floating gate 505 is formed on the sidewalls 506 of a trench 507. Gatedielectric film 508 is formed between floating gate 505 and sourceregion 502, drain region 504, as well as channel region 503. Controlgate 509, formed adjacent to floating gate 505 in trench 507, coversfloating gate 505. The control gate 509 is insulated from floating gate505 and source region 502 by a layer of interpolysilicon dielectric film510.

The transistor channel width of cell 500 is equal to the perimeter oftrench 507 as defined by the trench opening on the surface of the chip,provided that sidewalls 506 of trench 507 are formed anisotropically anddrain region 504 is formed on all edges 511 of trench 507. Transistorchannel length 512, which is the distance between source region 502 anddrain region 504, is determined by the thickness of channel region 503rather than trench depth 513. To ensure channel conductivity, the bottom514 of trench 507 extends to source region 502. Cell 500 is programmedby conventional hot electron injection and is flash erased by electrontunneling from floating gate 505 to either source region 502 or drainregion 504.

2. Vertical Stacked Gate EEPROM with Separate Tunnel Oxide Layer

In another embodiment of the present invention shown in FIG. 6, avertical stacked gate EEPROM cell 600 has a tunnel oxide layer 615formed on the top surface of drain region 604. Transistor 600 isprogrammed by conventional hot electron injection and flash erased byelectron tunneling from floating gate 605 to drain region 604. Becausefloating gate 605 now overlies drain region 604 between the trench 607and bit line isolation 616, all tunneling during erasing occurs throughtunnel oxide layer 615. As a result, the thickness of gate dielectricfilm 608 is increased relative to the thickness of tunnel oxide layer615. This thicker gate dielectric film 608 provides significantimprovements in reliability and yield when compared to, for example,prior art cells 200, 300 having thin gate isolation layers 204 and 304,respectively, which completely cover the channel region. The thicknessof gate oxide 608 on channel region 603 is typically 10 to 30 nanometersthicker than gate oxide 508 (FIG. 5). FIG. 6 illustrates across-sectional view of EEPROM cell 600 along a word line in the array.The cross-sectional view of transistor 600 along a bit line is identicalto that of the vertical stacked gate memory cell 500 shown in FIG. 5A.

3. Vertical Stacked Gate EEPROM with Extended Drain Region

FIGS. 7A and 7B show cross-sectional views of a vertical stacked gateEEPROM cell having an extended drain region 717 along a word line and abit line, respectively. This extended drain region 717, typically formedfrom polysilicon, provides a tunneling electrical field enhancementeffect. It is well known to those skilled in the art that oxide grownthermally on a pretreated polysilicon surface (polysilicon heavily dopedwith an N+ dopant) provides a localized enhanced electrical field acrossthe oxide, thereby lowering the voltage required to be applied acrossthe oxide to achieve electron tunneling compared to the applied voltagesrequired for oxide grown on a monocrystal silicon surface. In otherwords, the polysilicon surface of extended drain region 717 allows theoxide, i.e., gate dielectric film 708, to be grown thicker withoutincreasing the voltage to achieve electron tunneling. Typically, gatedielectric film 708 is grown to a thickness between 20 and 40nanometers. Because of the increased thickness of gate dielectric film708, cell 700, like cell 600, significantly reduces prior artmanufacturing yield problems. Memory cell 700 is programmed byconventional hot electron injection and flash erased by electrontunneling from floating gate 705 to extended drain region 717.

4. Vertical Stacked Gate EEPROM with Separate Erase/Program Gate

In another embodiment of the present invention, shown in FIGS. 8A and8B, memory cell 800 is erased by electrons tunneling from floating gate805 to a separate erase/program gate 818. Memory cell 800 is programmed,in contrast, by electrons tunneling from erase/program gate 818 tofloating gate 805. A dielectric film layer 819, typically a siliconoxide having a thickness of 30 to 50 nanometers, is provided toelectrically isolate drain region 804 from erase/program gate 818.Providing an appropriate voltage bias to memory cell 800, such as -6 Vto -12 V on control gate 809 and +5 V to +8 V on erase/program gate 818,erases memory cell 800 without disturbing the unselected memory cells.In a similar manner but using the opposite polarities of the voltagebias condition described above, memory cell 800 is programmed byelectrons tunneling from erase/program gate 818 to floating gate 805.Therefore, by using erase/program gate 818, cell-by-cell erasability isachieved.

5. Vertical Split Channel EEPROM with Separate Erase/Program Gate

In accordance with another embodiment of the present invention, shown inFIGS. 9A and 9B, a series select transistor 922 is included in verticalsplit channel memory cell 900, thereby expanding the use of memory cell900 in the depletion mode and eliminating potential read errors causedby an over-erased floating gate. Series select transistor 922 isintegrated into the split channel EEPROM cell structure by addinganother trench 920 adjacent to trench 907, thereby forming a splitchannel EEPROM structure. FIGS. 9A and 9B show cross-sectional views ofvertical split channel EEPROM cell 900 with separate erase/program gate918 along a word line and a bit line, respectively.

In this embodiment, two transistors are formed in series: transistor 921formed in trench 907 and transistor 922 formed in trench 920. Transistor921 is the floating gate transistor with channel length 912 determinedby the edge of drain 904 and the bottom of trench 914 and with thechannel width determined by the perimeter of trench 907. Transistor 922is the series select transistor with channel length 923 determined bythe distance between the bottom of trench 914 and source region 902, andwith the channel width determined by the perimeter of the second trench920.

Note that trench 920 is self-aligned to the inner edges of floating gate905 inside trench 907. Control gate 909 of floating gate transistor 921extends through trench 920 to source region 902. Control gate 909 is inoperative relation to channel region 903B of transistor 922 and isinsulated from channel region 903B and source region 902 by a layer 924of gate oxide having a thickness of 20 to 40 nanometers. If seriesselect transistor 922 is off, no current flows between source region 902and drain region 904 even if floating gate transistor 921 is in thedepletion mode. Note that channel region 903 is thicker than, forexample, channel region 803 (FIGS. 8A and 8B) to accommodate theadditional trench 920. However, as shown in FIGS. 9A and 9B, the presentinvention provides a vertical split channel EEPROM cell 900 having aseparate erase/program gate 918 and a select transistor 922 withoutrequiring additional surface area on the chip.

II. Methods of Manufacturing Vertical EEPROM Cells

Typically, a plurality of memory cells are grouped together to form thememory core of a semiconductor storage device. Other circuitssurrounding the memory core perform functions such as decoding,accessing, memory erasing, programming, sensing, and data buffering. Dueto some relatively high voltage circuits on chip, planar thick gateoxide MOSFETs (i.e. having gate oxide thickness in the range of 35 to 65nanometers) are needed in addition to conventional planar MOSFETs (i.e.having gate oxide thickness in the range of 15 to 35 nanometers). Themethods of manufacturing EEPROM cells in accordance with the presentinvention are compatible with the manufacturing of both types ofMOSFETs.

1. Vertical Split Channel EEPROM with Separate Erase/Program Gate

In accordance with the present invention, the transistor trench iseither non-self-aligned or self-aligned to the edge of the bit lineisolation. In the non-self-aligned configuration, all four edges of thetrench are defined by a photomask. The distance between the trench edgeand the edge of the bit line isolation is larger than the sum of theminimum drain width required for each side of the trench and the minimumlithographical alignment tolerance. In the self-aligned configuration, aspacer formed on the sidewall of the bit line isolation defines twoedges of the trench parallel to the bit line. Thus, the trench edgesparallel to the bit line are self-aligned to the bit line isolation andthe minimum lithographical alignment tolerance is not required to bepart of the spacing between the trench and the bit line isolation.

FIGS. 10A through 10E illustrate cross-sectional views along a word lineof two vertical split channel EEPROM cells having separate erase/programgates and non-self-aligned trenches at various stages in fabrication.Referring to FIG. 10A, a P-type single crystal silicon substrate 1001 islightly doped with boron to a resistivity ranging from 10 to 60ohms-centimeter. A heavily doped N+ buried layer 1002 is formed onsubstrate 1001 in the memory core area and in the N-well area in theperiphery of the memory device. Preferably, buried layer 1002 has a lowsheet resistance of, for example, 20 to 150 ohms per square. This lowsheet resistance minimizes the number of ground pick-ups in the memorycore and ensures that the potential of buried layer 1002 (also referredto as the source region) is not raised during cell operations.

A layer 1003 of silicon epitaxial film approximately 1.5 to 3micrometers thick is then deposited on top of N+ buried layer 1002 andP-type single crystal silicon substrate 1001. In one embodiment, layer1003 is very lightly N-doped to a resistivity higher than 1ohm-centimeter. After epitaxial deposition, conventional well formationin the array and the periphery is performed. The P-wells in the memorycore (from which the channel regions will be formed) typically have adoping concentration of about 1×10¹⁶ cm⁻³ to 2×10¹⁷ cm⁻³, whereas theP-wells in the periphery have a doping concentration consistent withconventional planar N-channel MOSFETs of about 5×10¹⁵ cm⁻³ to 2×10¹⁶cm⁻³. The N-wells in the periphery have a doping concentrationconsistent with conventional planar P-channel MOSFETs of about 8×10¹⁵cm⁻³ to 4×10¹⁶ cm⁻³.

Following well formation, the memory core area is covered with a siliconnitride layer (not shown) to prevent oxide from growing in this areaduring the subsequent peripheral field oxidation. Conventional localoxidation of silicon (LOCOS) is then used to form isolated channelregions for the peripheral planar MOSFETs. The silicon nitride layer isthen etched away in a conventional manner.

To form the drain regions of the to-be-formed vertical EEPROMtransistors, arsenic is implanted in the memory core area at a dose of5×10¹⁵ cm⁻² to form layer 1004. A typical thickness of layer 1004 afterfull process completion is about 250 to 400 nanometers. Layer 1004 isthen thermally oxidized to grow a layer of oxide 1019 about 35 to 65nanometers thick followed by deposition of a heavily N+ dopedpolysilicon layer 1018 to a thickness of about 150 to 400 nanometers.Layer 1019 isolates layer 1004 from polysilicon layer 1018, and alsoserves as the gate oxide of the MOSFETs in the periphery.

Photoresist layer 1025 is deposited on polysilicon layer 1018 and thenpatterned. Subsequent to patterning, polysilicon layer 1018, oxide layer1019, and N+ doped layer 1004 are anisotropically etched as shown inFIG. 10A. This etching defines the bit lines, i.e. drain regions, whichare formed from layer 1004. Layer 1018 provide the erase/program gatesof the EEPROM cells in the array, as well as the gate electrodes for theperipheral MOSFETs.

In one embodiment, a low dose boron implantation on partially-exposedlayer 1003 ensures proper isolation between adjacent bit lines. Bit lineisolation is completed by depositing a layer 1027 of CVD oxide on thearray to a thickness of about 800 to 1000 nanometers. Layer 1027 is thenplanarized using a conventional resist planarization and etch back toleave an oxide layer 1010 (about 100 to 200 nanometers) on polysiliconlayers 1018. In another embodiment, a LOCOS method is used to form theisolation for the bit lines. Photoresist layer 1026 is then deposited onthe array and patterned as shown in FIG. 10B to define the rectangulartrench opening 1007 (see FIG. 10C).

Vertical EEPROM transistor trenches 1007 are subsequently formed by ananisotropic etch through oxide layer 1010, polysilicon layer 1018, oxidelayer 1019, N⁺ doped diffusion region 1004, and epitaxial layer 1003.Trench 1007, in one embodiment of the present invention, is deep enoughto accommodate a floating gate transistor with a channel length rangingfrom 0.6 to 1.0 micrometers. Trench sidewalls 1006 are preferablyperpendicular to the surface of silicon substrate 1001 and have minimaletch damage left on them. The minimum spacing 1029 between trenchsidewall 1006 and bit line isolation 1027 is greater than the sum of thedrain width required per trench edge and the process toleranceassociated with the photomasking process.

A layer 1008 of thermal oxide having a thickness of about 20 to 40nanometers is grown on trench sidewalls 1006 and at the bottom of trench1007. In one embodiment, oxide layer 1008 is grown at a relatively hightemperature of, for example, 950°-1000° C. and in a dry oxygen ambientso that an enhanced oxidation caused by high dopant concentration indrain region 1004 and erase/program gate 1018 can be minimized.

After this oxidation process, a layer of heavily phosphorus doped N+polysilicon is deposited and anisotropically etched back without usingany mask to form polysilicon floating gate 1005. The thickness 1030 offloating gates 1005 is typically 50 to 200 nanometers. Then a layer 1031of interpoly dielectric film or a combination of dielectric films (suchas oxide and nitride) is deposited to a thickness of about 20 nanometerson the exposed surfaces of floating gates 1005.

Deposition of layer 1031 is followed by deposition of a layer 1032 ofphosphorus doped oxide (PDO). PDO layer 1032 is deposited in anyconventional non-conformal manner such that the thickness of PDO layer1032 deposited on the bottom and the sidewalls of trench 1007 is muchthinner than the thickness of PDO layer 1032 deposited on the topsurface of the array. In this manner, a plasma etch of PDO layer 1032exposes the bottom of the first trench leaving the top surface and thesidewalls unexposed. After dry etching removes interpoly dielectric film1008 at the bottom of trench 1007 and exposes layer (hereinafter channelregion) 1003 a highly selective (i.e., to oxide) anisotropic siliconetch through channel region 1003 forms another trench 1020 as shown inFIG. 10D. Note that the bottom of trench 1020 must reach source region1002 to ensure proper channel conductivity of the memory cells. Thedepth of trench 1020 is typically in the range of 0.75 to 1.0micrometers so that the channel length 1023 of the to-be-formed seriesselect transistor is approximately 0.5 to 0.75 micrometers.

After formation of trench 1020, a photoresist mask (not shown) is usedto define the gate electrodes of the thick oxide MOSFETs in theperiphery. This step also removes dielectric layer 1031, polysiliconlayer 1018, and oxide layer 1019 from the thinner oxide MOSFET region inthe periphery. Following photoresist removal, gate oxide 1024 is grownover the sidewalls of trench 1020 and the thin oxide MOSFETs in theperiphery. The thickness of this gate oxide is typically in the range of20 to 40 nanometers. Then, a layer 1009 of heavily N+ doped polysilicon(or a composite layer of silicide/N+ polysilicon) is deposited on thearray to fill trenches 1007, 1020 and cover the array as shown in FIG.10E. This polysilicon layer 1009 is patterned to form the control gatesof the memory cells, i.e., the word lines of the array (see FIG. 11), aswell as the gate electrodes of the thin oxide MOSFETs in the periphery.

After conventional source and drain formation in the periphery, a thicklayer of glass, usually PDO or borophosphosilicate glass (BPSG), isdeposited and patterned for contact openings. Conventional metallizationprocesses are carried out thereafter to interconnect all memory cellsand MOSFETs in the periphery.

FIG. 11 shows a top view of one embodiment of a 2×2 array 1100 ofvertical EEPROM cells 1101, each cell 1101 having an associatednon-self-aligned trench 1007. Note that array 1100 may include anyvertical EEPROM cells in accordance with the present invention. Aspecific memory cell is accessed in a conventual manner, i.e. byselecting the word line 1009 and bit line 1004 associated with thatcell. Cells 1101 and their corresponding trenches 1007 are located atthe intersections of word lines 1009 and bit lines 1004. Theerase/program lines 1018 are formed on top of bit lines 1004 and sharethe identical layout with bit lines 1004.

For a given technology with minimum feature size X, the area A_(cell) ofany memory cell of the array is determined by the following equation:

    A.sub.cell =4X.sup.2 +4XS

where S is the spacing between the edge of trench 1007 and the edge ofbit line isolation 1027. More specifically, S is the sum of the minimummisalignment tolerance and the minimum drain width required per trenchedge. Because the trench edges are defined lithographically, thesmallest possible dimension the trench edge has is the minimum featuresize X. For example, in 1.0 μm technology, X=1.0 μm and S=0.5 μm yieldsan area A_(cell) equal to 6 μm², which is significantly smaller than anyprior art split channel EEPROM cell having a separate erase/programgate.

To further reduce cell size, two edges of the trenches are self-alignedto the bit line edge. The process steps to provide these self-alignedtrenches are identical to those steps described above in reference toFIGS. 10A-10E for cells having non-self-aligned trenches with thefollowing differences noted in reference to FIGS. 12A-12D.

After deposition of polysilicon layer 1218, a layer 1233 of PDO about400 to 800 nanometers thick is deposited on polysilicon layer 1218. Aphotoresist layer 1225 is then formed and patterned to etch PDO layer1233, polysilicon layer 1218, oxide layer 1219, and drain region 1204 asshown in FIG. 12A. CVD oxide 1227 is deposited on the array to athickness of 800 to 1000 nanometers. Using a conventional resistplanarization technique, CVD oxide 1227 is etched back to expose PDOlayer 1233. After this oxide etch, the thickness of PDO layer 1233should be in the range of 250 to 450 nanometers on polysilicon layer1218.

Next, PDO layers 1233 are selectively etched away using hydrofluoricacid to expose sidewalls 1234 of bit line isolation 1227. This etch isfollowed by formation of oxide spacers 1235 formed on sidewalls 1234 asshown in FIG. 12C. During the subsequent trench formation, the sides1336 of trench 1207 are defined by oxide spacers 1235 as shown in FIG.12D. The other two sides 1337 of trench 1207 (see FIG. 13) are definedby a photomask (not shown). The remaining steps, including the formationof gate isolation 1208 and floating gate 1205, are the same asnon-self-aligned scheme described in reference to FIGS. 10A-10E.

The use of oxide spacer 1235 to define trench edges 1336 eliminates anypotential misalignment problem. Furthermore, because trench edges 1336are not defined lithographically, trench width 1338 in the word linedirection can be made smaller than the minimum feature size allowedlithographically. Thus, the spacing S (FIG. 13) (referenced as spacing1229 in FIG. 12) between trench edge 1206 and bit line isolation 1227,now determined by the width of oxide spacer 1235, is significantlyreduced.

FIG. 13 shows a top view of a 2×2 cell array 1300 of vertical EEPROMcells 1301 having self-aligned trenches 1207. As mentioned previously,this array configuration is applicable to all vertical EEPROM cells inaccordance with the present invention. The bit line width, defined by aphotomasking process, is equal to the minimum feature size allowed, X.The trench width 1338 along the word line direction is equal to X=2S,where S is the oxide spacer width. The area of the memory cell A_(cell)is equal to 4X². For 1.0 μm technology, X is equal to 1.0 μm. Thus, thecell area A_(cell) is equal to 4 μm², which is a 33% reduction in cellsize compared to the size of a cell having a non-self-aligned trench(See FIG. 11). However, because the width W of the memory cell issmaller, the trench cell current is reduced. Specifically, the trenchperimeter is now equal to 4X-2S instead of 4X. The spacing betweentrench edge and bit line edge, S, can be adjusted by varying the spacerwidth and is equal to the minimum drain width required per trench edge.

Note that vertical EEPROM cells in accordance with the present inventionare highly scalable. Therefore, cell size is reduced in proportion tothe minimum feature size reduction. For example, if the minimum featuresize is reduced 20% from 1.0 μm to 0.8 μm, then the cell size iscorrespondingly reduced 36% from 4 μm² to 2.56 μm².

2. Process Modifications

Those skilled in the art will recognize that elimination or addition ofone or more steps in the above-described process descriptions provides avertical EEPROM with different features. For example, to provide avertical stacked gate EEPROM with a separate erase/program gate, thethickness of the channel region is reduced to about 1.2 to 2.0micrometers such that the bottom of the trench reaches the sourceregion. Those steps associated with the series select transistorformation are deleted.

As another example, to provide only vertical EEPROM cells without otherfeatures, the polysilicon layer used as the erase/program gate isreplaced by a layer of oxide or by a nitride film used in a LOCOSprocess. As a further example, to produce vertical EEPROM cells havingextended drain regions, the step of depositing oxide underneath thepolysilicon layer is eliminated so that the polysilicon layer is incontact with the drain region and hence becomes the extended drainregion.

Finally, to produce a vertical split channel EEPROM with a tunnel oxideformed on the top surface of the separate erase/program gate, a numberof maskless process steps are added to the basic process as explained inreference to FIGS. 14A-14E. After deposition of polysilicon layer 1418,tunnel oxide 1434 is grown on the top surface of polysilicon layer 1418to a thickness of approximately 10 to 25 nanometers. Then, a layer 1435of polysilicon is deposited to a thickness of about 100-250 nanometers.An anisotropic etch using patterned photoresist layer 1425 yields thestructure illustrated in FIG. 14A.

Oxide 1427 is then deposited over the array and planarized back to formthe bit line isolation and leave a layer of oxide 1436 of approximately1,000 Å on top of polysilicon layer 1435 as shown in FIG. 14B.Photoresist layer 1426 is formed and patterned to define the trenchopenings. An anisotropic etch forms trenches 1407 as illustrated in FIG.14C. In conformance to the standard process steps described above, agate dielectric film 1408 is subsequently thermally grown on trenchsidewalls 1406.

Then, a layer 1437 of polysilicon is deposited to protect gatedielectric film 1408 during the following processing steps. After thispolysilicon deposition, a layer of photoresist is spun on the wafer andis exposed without a photomask such that only trenches 1407 are filledwith photoresist 1438 as shown in FIG. 14C. The exposed portions ofpolysilicon layer 1437 and the underlying silicon oxide layer 1436 arethen removed. A wet oxide etch step is carried out before the removal ofphotoresist to partially etch away the oxide between layer 1435 andlayer 1437. This step creates a gap 1440 between polysilicon layers 1435and layer 1437. Subsequently, a layer 1439 of polysilicon having athickness of about 50 to 150 nanometers is deposited. This polysiliconlayer 1439 fills in gap 1440 and connects layer 1435 to layer 1437. Theremaining process steps to obtain the structure shown in FIG. 14E arethe same steps described in reference to FIGS. 10C-10E, for example.

3. Vertical EEPROM Structures Using N+ Silicon Substrate

In another embodiment of the present invention shown in FIG. 15, avertical EEPROM cell is formed on a standard N-type CMOS epi substratewhich is generally less expensive compared to the custom epi substratedescribed above in reference to FIGS. 5-14. An N-type CMOS epi substrateis typically an intrinsic or lightly doped N-type silicon epitaxiallayer 1503 deposited on an antimony doped N+ monocrystal siliconsubstrate 1540. FIG. 15 shows a cross-sectional view along a word lineof a vertical split channel EEPROM cell 1500 with separate erase/programgate 1518 formed on a standard N-type CMOS silicon epi substrate. Allprocessing steps are identical to those described previously inreference to FIGS. 10A-10E with the exception that buried layer sourceregion 1002 and substrate 1001 are replaced by the N⁺ monocrystalsilicon substrate 1540. However, because N+ substrate 1540 is used asthe common source region in the memory core as well as the N-well forthe planar p-channel MOSFETs in the periphery, N+ substrate 1540 isconnected to the most positive potential on chip. Thus, the potential ofthe common source region of the memory cells cannot be grounded. The bitline voltage, i.e. the drain voltage, is now lower than the sourcevoltage. Thus, memory cell 1500 is sourcing current during the readoperation instead of sinking current as is typical during a commonlygrounded source approach.

4. Memory Cell Operations

The key operations for any memory device are write and read, which allowdata to be stored into and retrieved from a storage element,respectively. The write operation of a conventional EEPROM cell,especially a flash-EEPROM cell, generally includes two steps asmentioned briefly above: erasing the cell by discharging the floatinggate, and programming the cell by charging the floating gate. Becauseflash-EEPROMs cannot be erased cell-by-cell, these memory cells areusually first erased and then programmed to a fixed threshold voltage.In one embodiment of the present invention, the memory cells areprogrammed by hot electron injection in which the floating gate attractselectrons from the channel region near the drain diffusion and flasherased by electrons tunneling from the floating gate to the commonsource region.

FIG. 16 is a schematic diagram of a 2×2 array 1600 of vertical splitchannel EEPROM cells 1601. In addition to the commonly grounded P-wellregion 1608 and the source region 1609, there are three other terminalsassociated with each memory cell 1601: control gate 1602, erase/programgate 1603, and drain 1604. The voltage applied on each of these threeterminals is adjusted independently to access and determine theoperation mode of each memory cell 1601. The word line 1605 (connectingall the control gates 1602 of memory cells 1601 on the same row) and thebit line 1606 (connecting all the drains 1604 of memory cells 1601 onthe same column) are conventionally used to select a specific memorycell 1601 during a read operation. Because all erase/program gates 1603in the same column are connected, a memory cell 1601 is selected duringa write operation, i.e. for erasing or for programming, by choosing aword line 1605 and erase/program line 1607.

FIG. 17 illustrates a timing diagram of voltages on control gate 1602(V_(CG)), drain 1604 (V_(D)), and erase/program gate 1603 (V_(E/P))during various operations. The bias conditions for all cells in thearray, selected and unselected, are summarized below in Table I:

                  TABLE I                                                         ______________________________________                                                              Same Row  Same Column,                                            Selected    Unselected                                                                              Unselected                                    Read Mode Cell        Cell      Cell                                          ______________________________________                                        V.sub.CG  5 V         5 V       0 V                                           V.sub.D   2 V         0 V       2 V                                           V.sub.EP  0 V         0 V       0 V                                           ______________________________________                                                              Same Row  Same Column                                             Selected    Unselected                                                                              Unselected                                    Erase Mode                                                                              Cell        Cell      Cell                                          ______________________________________                                        V.sub.CG  -12 V       -12 V     5 V                                           V.sub.D   0 V         0 V       0 V                                           V.sub.EP  6 V         -5 V      6 V                                           ______________________________________                                                              Same Row  Same Column                                             Selected    Unselected                                                                              Unselected                                    Program Mode                                                                            Cell        Cell      Cell                                          ______________________________________                                        V.sub.CG  12 V        12 V      -5 V                                          V.sub.D   0 V         0 V       0 V                                           V.sub.EP  -6 V        5 V       -6 V                                          ______________________________________                                    

For example, in one embodiment of the present invention, the tunnellingoxide thickness is adjusted such that significant tunnelling occurs whenthe voltage across the tunnel oxide is equal to or greater than 12volts. Assuming the coupling ratio between control gate 1602 andfloating gate 1610 is 1/2, the tunneling condition is created, forexample, by providing 12 volts on control gate 1602 (via word line 1605)and -6 volts on erase/program gate 1603 (via erase/program line 1607).It is well known in the art that the voltage across the tunnel oxide isthe difference between the floating gate voltage (i.e. one half thecontrol gate voltage V_(CG) if the floating gate is uncharged) and thevoltage V_(EP). Depending on the polarity of the voltage applied oncontrol gate 1602 and erase/program gate 1603, the selected memory cellis either erased or programmed, i.e. the floating gate is eitherdischarged or charged, respectively. In this manner, cell-by-cellerasability and programmability are achieved. In order to prevent writedisturbing on unselected cells, especially those with negatively chargedfloating gate, unselected word lines and E/P lines are biased atpredetermined conditions such that the leakage current across the tunneloxide of unselected memory cells is negligible. In accordance with oneembodiment of the present invention, during the erase operation, +5 V isapplied on unselected word lines, i.e. control gate 1602, and -5 V isapplied on unselected E/P lines, i.e. erase/program gate 1603; and viceversa during the program operations.

To read a selected memory cell, a low voltage of, for example, 2.0volts, is provided to drain 1604 via bit line 1606 while groundingerase/program gate 1603 via erase/program line 1607. In anotherembodiment of the present invention, bit line 1606 is set at a fixedvoltage, e.g. 2.0 volts, throughout all operations so that the selectedmemory cell is continuously monitored during the program operation.

For those vertical EEPROM cells without a separate erase/program gate, amemory cell is selected by choosing a bit line and a word line for bothread and write operations. The bias conditions are summarized below inTable II:

                  TABLE II                                                        ______________________________________                                                              Same Row  Same Column                                             Selected    Unselected                                                                              Unselected                                    Read Mode Cell        Cell      Cell                                          ______________________________________                                        V.sub.CG  5 V         5 V       0 V                                           V.sub.D   2 V         0 V       2 V                                           V.sub.S   0 V         0 V       0 V                                           ______________________________________                                                              Same Row  Same Column                                             Selected    Unselected                                                                              Unselected                                    Erase Mode                                                                              Cell        Cell      Cell                                          ______________________________________                                        V.sub.CG  -12 V       -12 V     0 V                                           V.sub.D   Floating    Floating  Floating                                      V.sub.S   5 V         5 V       5 V                                           ______________________________________                                                              Same Row  Same Column                                             Selected    Unselected                                                                              Unselected                                    Program Mode                                                                            Cell        Cell      Cell                                          ______________________________________                                        V.sub.CG  12 V        12 V      0 V                                           V.sub.D   6 V         0 V       6 V                                           V.sub.S   0 V         0 V       0 V                                           ______________________________________                                    

Note that the bias conditions are similar to those for conventionalflash-EPROM cells. During an erase operation, electrons are transferredfrom the floating gate to the source region. Cell by cell erasiality isnot achievable for structures without a polysilicon erase/program gate1607.

5. Using On Chip Memory Array As Programmable References for Reading andWriting EEPROM Cells

As mentioned previously, the key functions of a memory device are torecord or store signals (i.e., information) and then playback orretrieve this information. Input and output signals are either analog ordigital signals.

Due to the advancement in high speed digital computation andtelecommunication, analog signals, such as audio and video signals, areconverted to digital signals for data processing and then laterconverted back to analog signals for audio playback or video/graphicaldisplay. This cycle of analog-to-digital conversion, digital data(signal) processing, and then digital-to-analog conversion is verycommon in many multimedia applications. In fact, data storing andretrieving are typically required before or after any digital dataprocessing.

Storing multiple bits of information in a single memory cell isequivalent to storing a sampled-and-held analog signal in a singlememory cell. The number of bits allowed to be stored per cell isdetermined by the resolution of the analog-digital conversion integratedon the same chip. Because the storage memory cells vary with temperatureand power supply fluctuations, the present invention provides a set ofreferences that tracks these cells for on-chip read and writeoperations. In accordance with the present invention, a plurality ofmemory cells in the array are prewritten to a determined thresholdvoltage and cell current level to function as references.

Allocating a plurality of memory cells to function as reference cellsprovides excellent tracking between storage cells and reference cellsbecause the transistor characteristics over both temperature and powersupply variations are identical for both storage cells and referencecells. These reference cells provide the limits (either voltage or cellcurrent levels) for characterizing state levels of each memory cell.Each memory cell storing n bits of digital data or equivalent analogsignal has 2^(n) associated states.

Each state is defined by a distinctive level of threshold voltage, orcell current, or even bit line voltage of a cell during a read mode.Each state is characterized in accordance with the present invention bya pair of limits, an upper limit and a lower limit, which constitute thereference levels. For example, as shown in FIG. 18, two bits of datastored in a memory cell are represented by four states or levels, S1 toS4. These four states are characterized by five reference levels, Ref 0to Ref 4.

State S1, representing the lowest level of data "00", is defined in oneembodiment as the lowest level of channel conductivity (whichcorresponds to the highest level of threshold voltage and the lowestlevel of cell current). State S1 is characterized by reference levelsRef 0 and Ref 1. To maximize the number of bits stored per cell, thedifference between each state (in terms of voltage or cell current),typically referred to as the least significant bit (LSB), is uniform andis equal to 2 times the maximum resolution limit. Reference levels areset at + 1/2 LSB and - 1/2 LSB from each state. Thus, each referencelevel is 1 LSB from the next reference level and 1/2 LSB from the nextstate. In accordance with the present invention, reference cells sharingthe same bit line are written to the same reference level. Referencesused for reading or writing to a selected memory cell are read out fromreference cells that share the same word line as the selected memorycell. For instance, five column of memory cells are required for two-bitdata storage (four states) per memory cell.

Because a memory device retrieves and plays back stored information, oneembodiment of the present invention varies the charging and dischargingof the floating gate by comparing the input information to the memorycell with feedback from this cell. For each digital input signal, a pairof references corresponding to the state representing the input datasignal is selected, thereby setting lower and upper limits.

FIG. 19 illustrates schematically how the lower limit reference 1901 andthe upper limit reference 1902 vary the programming and erasing of aselected storage cell 1911. Lower limit reference 1901 is provided tothe positive input terminal of lower limit comparator 1903 whereas upperlimit reference 1902 is provided to the negative input terminal of upperlimit comparator 1904. A feedback signal from storage cell 1911 on line1918 is provided to the negative input terminal of lower limitcomparator 1903 as well as the positive input terminal of upper limitcomparator 1904. Lower limit comparator 1903 controls the activation ofthe erase operation via erase control 1905 while upper limit comparator1904 controls the activation of program control 1906. A total of 2^(n)+1 references, that is 2^(n) +1 columns of reference cells, are requiredfor writing n bits of data into a memory cell.

Using this writing scheme, the content of a selected memory cell, forexample cell 1911, is first compared with the new data to be stored asdefined by lower limit reference 1901 and upper limit reference 1902.Based on this comparison, either erase control 1905 or program control1906 is activated to respectively discharge (erase) or charge (program)floating gate 1915. This process repeats until both erase control 1905and program control 1906 are disabled by comparators 1903 and 1904,thereby signifying the state of the selected memory cell is at the samelevel as the input signal. In this manner, the inefficient, prior artmethod of fully erasing the selected storage memory cell beforeprogramming it to a new state is eliminated. Moreover, because theprogramming and erasing of selected memory cell 1911 is repeated untilthe feedback signal on line 1918 from storage cell 1911 matches thestate of the input signal, the conventional program/erase windownarrowing effect has significantly less impact on the writing methodused in the present invention.

For an analog input of level Sa, instead of converting the input to adigital format and then selecting the associated pair of references, theupper limit reference 1902 and lower limit reference 1901 are changed toSa+ 1/2 LSB and Sa- 1/2 LSB, respectively. The least significant bit(LSB) is the difference between two adjacent reference levels and isgenerated on chip with the same set of references as described above inreference to FIG. 18.

Memory cell 1911 shown in FIG. 19 is a vertical split channel EEPROMcell having a separate erase/program gate 1916. If memory cells withouterase/program gates are used, then signals from program control 1906 anderase control 1905 are provided on bit line 1914. If flash-EEPROM cellsare used, a block of memory cells are erased simultaneously prior toprogramming. Then, the feedback method described in detail above inreference to FIG. 19, together with the use of a minimum programmingpulse, set the memory cell level to just above the desired lower limit.

In accordance with the present invention, if a defective memory cell isselected and is unable to achieve the desired state despite charging ordischarging the floating gate, a maximum write limit is establishedon-chip to stop the write operation. The maximum write limit is either areal time limit or a limit on the number of program/erase pulses appliedto the memory cell. Once either write limit is reached, the writeprocess is stopped and directed to continue on a redundant cell providedin the array.

To read stored information as an analog output signal from a selectedmemory cell, the memory cell current is provided to one input terminalof a differential amplifier. A reference level of maximum channelconducting is provided to the other input terminal of the differentialamplifier to compensate for variations in the temperature and the supplyvoltage. The output signal of the differential amplifier is the analogoutput of the memory cell. Referring to FIG. 20, to read storedinformation in cell C₁ as a digital output signal, comparators 2000₁through 2000_(N) compare the cell current from cell C₁ with that ofon-chip reference cells C_(REF1) through C_(REFN). Encoder 2001 encodesthe result of this comparison into digital data. Note that the readoutof cell C₁, i.e. either the cell current or the corresponding bit linevoltage, is compared to the readout of the reference cells C_(REF1)through C_(REFN) in parallel. The number of references cells (and thenumber of comparators) required for n bits storage per memory cell is2^(n) -1. References below the lowest state and above the highest arenot needed to encode the comparison results into digital data.

To improve the read access time of a memory cell, one embodiment of thepresent invention senses the bit line voltage instead of the cellcurrent. It is well known in the art that the channel conductance of amemory cell is a linear function of the stored charge and thresholdvoltage, assuming the drain voltage is much smaller than the differenceof the gate voltages V_(G) and the threshold voltage V_(T). The cellcurrent will pull down the precharged bit line level when the selectedbit line is left floating during a read operation. Thus, the decay ofthe bit line voltage is solely a function of the cell current.

At the same time, the reference bit lines BL_(REF) are all selected (byturning on transistor T_(REF)) and the cell current of reference cellsC_(REF) associated with, for example, word line WL_(N) of selectedmemory cell C1 will pull down the voltage on bit lines BL_(REF) atdifferent rates. At time t_(s) after word line WL_(N) is turned on, thespectrum of reference levels provided by reference C_(REF) are providedto comparators 2000. The bit line voltage level of memory cell C1 isalso provided to comparators 2000 via bit line BL₁ at the same timeunder the same environment as reference bit lines BL_(REF). In otherwords, because memory cell C1 and reference cells C_(REF) are connectedto word line WL_(N), accurate tracking between the selected memory celland its associated reference cells under all chip conditions is ensured.The output signals from comparators 2000 are encoded by encoder 2001 toyield a digital readout of memory cell C1. The precharged bit line leveland the sampling time t_(s) are selected to maximize the storagecapacity per memory cell.

FIG. 21 illustrates a schematic block diagram of a system for storingmultiple bits per cell or analog signals. Address input signals, i.e.both row and column addresses, are first latched into address latch 2101and then directed into column decoder 2102 and row decoder 2103. Theoutput signal from row decoder 2103 activates the word line of theselected memory cell in memory array 2107 and the correspondingreference cells in reference array 2108. The output signal from thecolumn decoder 2102 activates the bit line and the erase/program line ofthe selected memory cell. The bit lines of the reference cells areselected by the signals from reference selection 2104, which decodes thedigital data from input buffers 2105. Write control 2106 consists of theerase control 1905 and program control 1906 (see FIG. 19). During thewrite operation, two reference cells in the reference array 2108 areselected according to the data input signals. The cell currents of thesetwo reference cells are fed into the limit comparators 2111 as the lowerand upper limits. The cell current of the selected memory cell in memoryarray 2107 is also directed into the limit comparators 2111 to comparewith the cell current of the two selected reference cells. The result ofthis comparison is fed back to the write control 2106 to set up thepredetermined bias condition for erase and program operations. Duringthe read operation, data comparators 2110 are used to compare the cellcurrent of the selected memory cell in memory array 2107 with that ofall reference cells in the reference array 2108 associated with theselected word line. The comparison result is then encoded by the encoder2109 to provide the digital signals representing the stored information.These signals drive the output buffers 2112 to provide the memory outputsignals. The present invention, unlike conventional memory devices, (1)uses the readouts from a block of memory cell array as references, (2)establishes a set of the references to characterize each state of thememory cell, (3) employs an innovative write scheme to ensure that eachcell is at the desired state and is read out correctly, (4) uses 2^(n)-1 comparators to compare the storage cell readout and the referencecell readouts in parallel, (5) uses an encoder to generate digitaloutput from the comparators' results, and (6) integrating digital-analogconvertors into conventional memory devices.

In accordance with the present invention, both manufacturing yield andreliability are significantly improved in comparison with conventionalEEPROM cells. Specifically, in conventional EEPROM cells, two majorproblems in the yield and reliability area are (1) the program and erasewindow narrowing due to electrons trapped in oxide and electron trapsgenerated by tunneling electrons through the thin tunnel oxide,(explained in detail by C. S. Jenq, et al., in a lecture entitled, "HighField Generation of Electron Traps in MOS Capacitors", presented duringSemiconductor Interface Specialist Conference, December 1977. Thepresent invention solves the window narrowing problem by using cellcurrent feedback and the program/erase method described above inreference to FIGS. 19-21. Specifically, in accordance with the presentinvention, the memory cell is not programmed or erased by a fixed numberof program/erase pulses but rather is continuously programmed or eraseduntil the cell readout matches the input state.

Those skilled in the art recognize that the program and erase windownarrowing effect is cumulative and is thus worsened each time moreelectrons travel through the tunnel oxide. To slow down the windownarrowing effect and prolong memory cell life, the number of electronspassing through the tunnel oxide must be reduced. In a conventionalflash-EEPROM cell array, a given memory cell must be fully erased andthen reprogrammed irrespective of whether the present state is differentfrom the desired state, thereby unnecessarily increasing the number ofelectrons passing through the tunnel oxide and generating extra electrontraps which shorten memory cell life. In accordance with the writemethod of this invention, the memory cell readout is compared with theinput signal. When the two match, no erasing or programming occurs, i.e.no electrons will pass through the tunnel oxide.

Moreover, each conventional erase and program operation swings thethreshold voltage of the memory cell from the lowest allowed value tothe highest allowed value. This swing maximizes the number of electronspassing through the tunnel oxide, thereby further shortening memory celllife. The present invention uses on-chip memory cells as references,thereby reducing the number of electrons required to pass through thetunnel oxide to change the memory cell from one state to another state.The resulting improvement in sensing resolution yields a smaller leastsignificant bit (LSB). Moreover, reducing the number of electronspassing through the tunnel oxide also significantly reduces the numberof traps generated in the tunnel oxide, thereby greatly minimizing theimpact of the program and erase window narrowing effect. Thus, thepresent invention improves the reliability of the memory cell andprolongs the life of the memory device.

The present invention solves the tunnel oxide problem by providingvertical memory cells having the configurations described, in referenceto FIGS. 5-9 for example. Specifically, by using a polysilicon layer asan extended drain region or as a separate erase/program gate, relativelythicker tunnel oxide is grown on the polysilicon surface in comparisonwith conventional memory cells. In another embodiment, tunnel oxide isgrown on the top surface of the drain area or the erase/program gate. Inthis manner, the gate oxide of the storage transistor is formed by aprocess independent from the tunnel oxide. Thus, thickening the gateoxide in relation to the tunnel oxide is easily achieved in the presentinvention.

7. Vertical EEPROM Cells Formed on Side Walls of An Open Trench

The vertical EEPROM cells described above are formed inside a trenchwith a rectangular or square opening (see, for example, FIGS. 11 and13). In these embodiments, a floating gate transistor is formed on theinner four side walls of the trench, while the drain region surroundsthe trench. The commonly grounded source region in the present inventionis buried inside the silicon substrate by forming an N+ diffusion areaprior to the epitaxial layer deposition. In this closed-trenchconfiguration, the vertical EEPROM cells provide the advantage of alarge transistor width equal to the perimeter of the trench opening.

To reduce the cost of manufacturing the above-described EEPROM cell,another embodiment of the present invention eliminates the epitaxiallayer deposition. Instead of forming vertical EEPROM cells on thesidewalls of rectangular or square trenches, vertical EEPROM cells areformed on the sidewalls of long open trenches. A partial cut-away of anarray 2240 of these vertical stacked gate EEPROM cells is shown in FIG.22. The long trenches 2207 run parallel to the drain region 2204. Sourceregions 2202 are formed at the bottom of trenches 2207 and are buriedunder floating gates 2205 but not directly under drain regions 2204.Source regions 2202 are formed between drain regions 2204 but at adifferent silicon surface level.

Fabrication of vertical stacked gate EEPROM cells having erase programgates formed on the sidewalls of long open trenches 2207 starts with aP-type single crystal silicon substrate 2201 with resistivity rangingfrom 30 to 60 ohm-centimeters. The steps for forming, for example, avertical split channel EEPROM having a separate erase/program gate (asshown in FIG. 10A) are followed with the exception of forming the buriedN+ layer 1002 and silicon epitaxial layer 1003. In this embodiment ofthe present invention and referring to FIG. 23A, long open trenches 2307are formed by etching through oxide layers 2310 and 2319, polysiliconlayer 2318 (erase/program gate), and N⁺ doped single crystal siliconlayer 2304 (drain region) into silicon substrate 2301. The depth oftrench 2307 is approximately 1.0 to 1.5 micrometers below the surface ofsilicon substrate 2301. Gate dielectric film 2308 having a thickness of20 to 40 nanometers is then grown on the sidewalls of these longtrenches 2207. After this gate oxide growth, floating gates 2305, shownin FIG. 23B, are formed on gate dielectric film 2308 by previouslydescribed spacer formation processes. The width of floating gate 2305 istypically in the range of 50 to 200 nanometers.

Next, arsenic dopants are implanted to form N+ diffusion source regions2302 at the bottom of trenches 2307. A layer of interpoly dielectricfilm or combination of dielectric films (ONO stacked films) with athickness equivalent to approximately 20 nanometers of oxide isdeposited over the array and inside trenches 2307. As shown in FIG. 23C,another layer of heavily phosphorus doped N+ polysilicon is thendeposited to fill trenches 2307 and patterned to form the control gates2309, i.e. word lines of the memory array. In another embodiment, thislayer of polysilicon deposited and used as control gate 2309 covers thesidewalls of trench 2307 but only partially fills trench 2307, therebysimplifying the subsequent self-aligned polysilicon etch step. Floatinggates 2305 are self-aligned to control gates 2309 and are etched in thesame etching step as the control gates (FIG. 22). Conventional standardCMOS EEPROM processes are then followed.

In accordance with another embodiment of the present invention andreferring to FIG. 23D, a split channel EEPROM cell having floating gates2305 formed on the sidewalls of long open trenches 2307 is formed byadding process steps prior to the last oxidation of the ONO formation.Specifically, the second long open trench 2320 is self-aligned to thefirst trench 2307 by depositing PDO and then anisotropically etchingback the PDO to expose the bottom of the first trench (see for examplethe description regarding FIG. 10D). FIG. 23D illustrates across-sectional view along a word line 2309 of a plurality of verticalsplit channel EEPROM cells formed on the sidewalls of open trenches (notshown).

The vertical EEPROMs produced in accordance with this embodiment arescalable and eliminate the need for an epitaxial layer deposition. FIG.24 shows a top view of the array 2340 of vertical EEPROM cells formed onthe sidewalls of long open trenches 2307. Note that in thisconfiguration, the trenches 2307 are formed between bit lines 2304 andintersect a plurality of word lines 2309. Two floating gate transistorsshare a drain region associated with each intersection of a word line2309 and a bit line 2304. For example, transistors T₁,1 and T₁,2 share adrain region (not shown) associated with the intersection of word line2309₁, and bit line 2304₁. Thus, a particular transistor, for exampletransistor T₁,2, is identified by the intersection of bit line 2304₁,word line 2309₁, and source line 2302₂. The cell size, the cell current,and the cell operation vary depending on the connection of source lines2302 as described in detail below.

Referring to FIG. 25A, if source lines 2302₁, 2302₂, and 2302₃ areconnected together and grounded, two transistors, for example T₁.1 andT₁.2, associated with the same drain region (see FIG. 23D) become onecell C1,1 having a floating gate transistor width equal to 2×W_(C),where W_(C) is the width of control gate 2309 (word line).

This common source connection makes the cell operation of these longtrench EEPROM cells identical to that of their counterparts described inreference to Tables I and II. The cell size of these long trench EEPROMcells C₁.1, C₁.2, C₂.1 and C₂.2 having a common source 2302 is equal to4X², where X is the minimum feature size of a given technology. For 0.8μm technology, the minimum feature size X is 0.8 μm and the cell sizeA_(cell) is equal to 2.56 μm².

If two source lines 2302₁ and 2302₂ associated with selected bit line2304₁ are connected to two separated global source lines SLB and SLArespectively as shown in FIG. 25B, two floating gate transistors T₁,1and T₁,2 associated with the intersection of word line 2309₁ and bitline 2304₁ constitute two memory cells C'₁,1 and C'₁,2 sharing one drainregion (not shown). In this embodiment, each cell C'₁,1 or C'₁,2 has afloating gate transistor width equals to W_(C). Thus, the cell size isnow one-half that of cells having a common source configuration (FIG.25A). For 0.8 μm technology, the minimum feature size X is 0.8 μm andthe cell size A_(cell) is equal to 1.28 μm².

Unlike the common source configuration shown in FIG. 25A, because twocells share the same drain region as well as the same erase/programgate, cell operations for the long trench EEPROM cells with thealternating source approach are different from those described inreference to Tables I and II. Typically, these cells are programmed byhot electron injection and erased by tunneling electrons from thefloating gate to the source region. The unselected source lines arefloating during all operations. Tables III and IV summarize the biasconditions for illustrative cells C'₁.1 and C'₁.2, respectively.

                  TABLE III                                                       ______________________________________                                        To program cell C'.sub.1.1 :                                                               2309.sub.1 = 12 volts                                                                        2304.sub.1 = 6 volts                                           2302.sub.1 = Ground                                                                          2302.sub.2 = Floating                             To read cell C'.sub.1.1 :                                                                  2309.sub.1 = 5 volts                                                                         2304.sub.1 = 2 volts                                           2302.sub.1 = Ground                                                                          2302.sub.2 = Floating                             To erase cell C'.sub.1.1 :                                                                 2309.sub.1 = -12 volts                                                                       2304.sub.1 = Floating                                          2302.sub.1 = 6 volts                                                                         2302.sub.2 = Floating                             ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        To program cell C'.sub.1.2 :                                                               2309.sub.1 = 12 volts                                                                        2304.sub.1 = 6 volts                                           2302.sub.1 = Floating                                                                        2302.sub.2 = Ground                               To read cell C'.sub.1.2 :                                                                  2309.sub.1 = 5 volts                                                                         2304.sub.1 = 2 volts                                           2302.sub.1 = Floating                                                                        2302.sub.2 = Ground                               To erase cell C'.sub.1.2 :                                                                 2309.sub.1 = -12 volts                                                                       2304.sub.1 = Floating                                          2302.sub.1 = Floating                                                                        2302.sub.2 = 6 volts                              ______________________________________                                    

Using these bias conditions, memory cells sharing the same global sourceline and word line will be erased simultaneously. In another embodiment,erasure is achieved by tunneling electrons from the floating gate to aseparate polysilicon erase gate instead of the source region. In thisembodiment, only cells C'₁.1 and C'₁,2 will be erased the same time.

The preceding description is meant to be illustrative only and notlimiting. Those skilled in the art will be able to devise otherstructures and methods within the scope of the present invention uponconsideration of the detailed description and the accompanying drawings.The present invention is set forth in the appended claims.

I claim:
 1. A method of forming a vertical memory cell comprising thesteps of:doping a substrate to form a source region; forming a channelregion on said source region; forming a drain region on said channelregion, wherein said drain region includes an extended drain region toprovide a tunneling electrical field enhancement effect; forming atrench through said drain region and said channel region; forming afloating gate in said trench; and forming a control gate in operativerelation to said floating gate.
 2. A method of forming a vertical memorycell comprising the steps of:doping a substrate to form a source region;forming a channel region on said source region; forming a drain regionon said channel region; forming a conductive layer insulated from saiddrain region; forming a trench through said conductive layer, said drainregion, and said channel region to said source region, thereby formingan erase/program gate from said conductive layer; forming a floatinggate in operative relation to said erase/program gate, said drainregion, said channel region, and said source region; and forming acontrol gate in operative relation to said floating gate.
 3. A method offorming a vertical memory cell, comprising:forming a first transistor ina first trench in a substrate; and forming a second transistor in asecond trench in said substrate, said second transistor being connectedin series with said first transistor in said memory cell, wherein saidsecond trench is disposed in said first trench and extends deeper intosaid substrate than does said first trench.
 4. The method of claim 3,further comprising:doping said substrate to form a source region;forming a channel region on said source region; forming a drain regionon said channel region; forming said first trench through said drainregion and a portion of said channel; forming a floating gate in saidfirst trench; forming said second trench through a portion of saidchannel region to said source region; and forming a control gate inoperative relation to said floating gate and said channel region,wherein said control gate is formed in said first and said secondtrenches.
 5. A method for forming a vertical memory cell,comprising:doping a substrate to form a channel region; depositing aconductive layer on said substrate to form a drain region; forming afirst trench through said conductive layer and into said substrate;forming a floating gate on the sidewall of said first trench; forming asecond trench in said substrate after forming said floating gate; dopingsaid substrate to form a source region in said second trench; andforming a control gate in operative relation to said floating gate,wherein said control gate is disposed in said first trench and saidsecond trench.
 6. The method of claim 5, wherein forming said secondtrench comprises forming said second trench at a bottom of said firsttrench.
 7. The method of claim 4 further comprising forming a conductivelayer insulated from said drain region, wherein said first trench isformed through said conductive layer, thereby forming an erase/programgate from said conductive layer.
 8. The method of claim 3, whereinforming said first transistor comprises forming a floating gatetransistor, and forming said second transistor comprises forming saidsecond transistor so that said first and second transistors bothmodulate a single channel region and together form a split-gate memorycell.
 9. A method for forming an array of vertical memory cellscomprising:forming a first layer of a semiconductor material having aconductivity type for forming channel regions of said memory cells;forming on said first layer, a second layer of a semiconductor materialhaving a conductivity type for forming drain regions of said memorycells; patterning said second layer to form bit lines; forming bit lineisolation regions between said bit lines and extending above said bitlines; forming spacers which are on sidewalls of said bit line isolationregions and overlying portions of said bit lines; and forming trenchesthrough said bit lines and into said first layer, said trenches havingsidewalls aligned with edges of said spacers.
 10. The method of claim 9,further comprising forming a source region, wherein said first layeroverlies said source region and forming said trenches creates openingsthrough said first layer to said source region.
 11. The method of claim9, further comprising doping bottoms of said trenches to from sourceregions for said memory cells.
 12. The method of claim 9, furthercomprising forming a third layer above said second semiconductor layer,wherein:patterning said second semiconductor layer comprises forming aphotoresist mask above said third layer, and etching openings throughsaid third layer and said second layer according to a pattern of saidphotoresist mask; forming said bit line isolation regions comprisesfilling said openings to a point approximately level with a top surfaceof said third layer; and selectively etching said third layer to createsidewall spacers on said bit line isolation regions.
 13. The method ofclaim 12, wherein said third layer comprises phosphorous doped silicondioxide, said bit line isolation comprises silicon dioxide, and saidselective etch is selective to phosphorous doped silicon dioxide. 14.The method of claim 9, wherein forming each of said trenchescomprises:forming a first trench in said first layer, wherein a sidewallof said first trench is aligned with an edge of one of said spacers;forming a floating gate on said sidewall of said first trench; andforming a second trench through a bottom surface of said first trench,said second trench having a sidewall aligned with an edge of saidfloating gate.
 15. The method of claim 9, further comprising:formingthird layer of a conductive material overlying said second layer; andpatterning said third layer to form erase program gates.
 16. The methodof claim 15, wherein:patterning said second layer uses a mask which isalso used when patterning said third layer; top surfaces of said bitline isolation regions are above a top surface of said third layer; saidspacers overlie portions of said erase/program gates; and forming saidtrenches etches through said erase/program gates and aligns edges ofsaid erase/program gates with said edges of said spacers.